The present invention relates generally to a method and apparatus for stacking non-symmetrical flexible articles. More particularly, it relates to a method and apparatus for stacking zippered plastic bags to maintain the top bag in a generally horizontal relationship.
In the production of individual flexible articles such as plastic bags and containers, zippered bag stock is typically supplied in the form of a continuous web of thermoplastic material having mating zipper elements near opposite edges thereof. The continuous web is folded upon itself to form two plies and to bring the zipper elements into opposing relationship. The folded web is thereafter severed to form individual plastic bags and containers from portions thereof. Typically, severing is achieved by use of hot wires, which simultaneously sever and seal the bag sides. The plastic bags are then stacked, counted, and packaged by packing equipment.
The severing and sealing operation typically takes place on a relatively large diameter rotating seal drum which includes a plurality of hot wire elements disposed in grooves along its outer periphery for severing and sealing. See, for example, Tumminia, U.S. Pat. No. 4,369,449, assigned to the same assignee as the present invention. As the seal drum rotates, the heated wires are sequentially raised to the surface to sever and seal a respective portion or the web and form a bag therefrom. The individual bags are retained on the seal drum by a vacuum arrangement as the seal drum rotates.
As the seal drum continues to rotate, the bags formed thereon are released and transferred to a smaller transfer drum, also typically equipped with vacuum capabilities. Transfer is accomplished by relieving the vacuum on the seal drum at an appropriate point to allow the bags to fall onto the transfer drum where they are, in turn, secured by vacuum. As the transfer drum rotates, its vacuum is relieved and the individual bags are stripped therefrom by an orbital packer or similar device. The orbital packer delivers the bags to a delivery point where they are stacked. Stacks of bags are thereafter transferred for packaging.
An orbital packer typically includes a plurality of packer fingers which move in a generally circular path in time with the transfer drum to remove successive bags therefrom for stacking. Typically, the first few bags of a stack are initially stacked on count fingers while the preceding stack is being removed. Thereafter, the stacking table, typically comprised of a plurality of fingers which nest with the count fingers, receives the bags from the count fingers. The count fingers retract and the stack continues to form on the stacking table. When a predetermined quantity of bags is reached, typically 25, the count fingers again fire into the stack to separate the stack just formed from the continuous stream of bags, allowing removal of the stack while the first several bags forming a new stack are received from the orbital packer on the count fingers. The process then continues to repeat, as described above. The stack formed on the stacking table is removed by a horizontal transfer mechanism, such as disclosed by Smith, U.S. Pat. No. 4,588,070, assigned to the same assignee as the present invention.
A number of problems have been encountered in the production of such thermoplastic bags. Stacking problems, such as quality and neatness of stacking, have long been experienced with thermoplastic bags on commercial, high speed stacking machinery, regardless of the manner of production. Bags have been difficult to stack due to their flexibility, and due to their slippery surfaces which are encouraged to aid in extrusion and film forming processes. Non-symmetrical flexible articles, such as zippered plastic bags, add to the difficulty with stacking. For example, a typical stack of 25 zippered plastic bags may be 2.0 inches or more thick at the zippered end (25 times the profile thickness), while being only 0.10 to 0.20 inches thick at the opposite end (25 times the bag film thickness). During stacking, the uppermost bags in the stack are frequently disturbed by the act of stripping bags from the transfer drum and depositing them onto the stack. Gravity and the impact of the bag being stacked typically cause the uppermost bags to slide down the stack towards the end opposite the zipper, producing a stack of shingled, rather than stacked, bags. In addition, where zippered bags are being stacked, due to the height of the stack formed thereby, the count fingers occasionally strike a zipper and attempt to drive the bag ahead, which in turn may crumple the bag and cause jamming.
Some attempts to solve this problem include using a fixed position, two-level stacking table with a lowered region at the zippered end of the stack. This type of table is limited to a height difference of only 1/4 inch to 1/2 inch, while the difference in stack height may be as much as 1.8-1.9 inches. The limitation arises because the stack is lowered onto the table when it is approximately half completed. If the height difference were any greater on such a two-level table, the stack might droop or fall through the table fingers and cause a jam.
Accordingly, the need remains for an improved apparatus and method for accurate high speed stacking of non-symmetrical articles.